Image rendering devices such as printers, scanners or copiers are widely used as output devices of computers. Take a color printer for example. For printing images by the color printer, a print command is issued from the host of the computer to the color printer. When the print command is received, the color printer may analyze and interpret the print command to generate a bitmap image. That is, when the print command is received by the raster image processor inside the color printer, the source image and text data are encoded in a specific page description language (PDL), for example a PostScript language provided by an Adobe System or a page control language (PCL) provided by Hewlett-Packard Company. As a consequence, the bitmap image is generated and stored in the raster memory of the color printer. The process of converting the analyzed data into the bitmap image is referred as a rendering method. By the rendering method, the CMYK color image or the RGB image received by the graphic device interface (GDI) of the host of the computer is converted into the bitmap image. After the whole page is rendered in the raster memory, the printer is ready for transmitting the rasterized dot stream to the printing paper.
During the image rendering process, the page description language (PCL) such as the page control language usually defines several raster operations (ROPs). In the PCL based systems, a total of 256 ROPs are defined. The ROPs specify the manner in which the source image, the paint and the destination image need to be combined to obtain the final bitmap image. The print controller of the color printer usually has a rendering engine to implement the image rendering process. In other words, the rendering engine may perform ROPs on the source image, the paint and the destination image. By the rendering engine, the essential display-list elements of the source image are combined as the bitmap image, thereby generating the paints for filling the source image. In addition, the rendering engine may perform ROPs on the source image and paint the destination image, thereby generating the bitmap image representing the page content.
Generally, the source image conforms to the CMYK or RGB standard. For example, in a case that the RGB image is 24-bit image, each channel has 8 bits. In views of the image rendering accuracy, an 8-bit operating engine is employed to perform ROPs on the whole source image, thereby generating accurate 8-bit image data. Next, the 8-bit image data are converted into 1-bit image data with different grayscale of brightness intensities according to a screening table. As known, this image rendering process needs very complicated computation and thus the rendering speed and efficiency are usually unsatisfied.
In another image rendering process, the source image is firstly converted into 1-bit image data and then the 1-bit ROP is done. Although the rendering speed and efficiency are enhanced, the image rendering accuracy is reduced.
Therefore, there is a need of providing an improved image rendering method for obviating the drawbacks encountered by the prior art.